This invention relates in general to processes of treating lignocellulosic materials and in particular to a process of producing a lignocellulosic pulp such as a wood pulp.
Lignocellulosic materials, such as wood, are plant materials made up primarily of cellulose, hemicellulose and lignin. The cellulose is the chief structural element and major constituent of the plants. It consists of a fibrous cellulose portion made from polymeric chains that are aligned with one another and form strong associated bonds with adjacent chains. The lignin is a three-dimensional polymeric material that bonds the cellulosic fibers and is also distributed within the fibers themselves. Lignin is largely responsible for the strength and rigidity of the plants. The hemicellulose is a polysaccharide that is a component of the cell walls of the plants.
The wood is converted to pulp for use in paper manufacturing. Pulp comprises wood fibers capable of being slurried or suspended and then deposited on a screen to form a sheet of paper. There are two main types of pulping techniques: mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are digested with chemical solutions to solubilize portions of the lignin and hemicellulose and thus permit their removal in the waste pulping liquor.
The commonly used chemical pulping processes include the kraft process, the sulfite process, and the soda process. The kraft process is the most commonly used and involves digesting the wood chips in an aqueous solution of sodium hydroxide and sodium sulfide. Environmental and economic limitations associated with the kraft process, however, have stimulated a search for alternative pulping processes.
Solvent pulping is an alternative to the kraft process in which delignification of wood chips is accomplished by fragmentation of the lignin by the dissolving action of solvent used in the cooking liquor. Low boiling temperature solvents are normally used in a solvent pulping process.